With support like that, who knows? It's not inherently sillier than an independent Scotland or an independent Catalonia anyway - or an independent Luxembourg, come to think of it.
Of course Italy was split into multiple nations until 150 years ago, but I would say the difference is that Catalonia and Scotland have particular identities clearly distinct from the whole bloc of their countries. Veneto might have strong peculiarities, but no more than, say, Apulia, Emilia or Campania. Italy is a construct of multiple regional identities, which is very different from relatively homogeneous blocs like UK and Spain where one or two specific regions clearly stand out.
I think trying to make comparisons like this is kind of futile. Is the difference between Veneto and Campania greater or lesser than the difference between Wallonia and Brittany? How about the difference between Quebec and Alberta? Or Mississippi and Vermont? Some of the differences between certain independent states are definitely much smaller than the differences between provinces/regions within certain states.
Different countries have different histories, and different levels of diversity. Some nations are highly ethnically and culturally homogeneous nation-states (Japan would be a good example). Others are federations of two or more arguably separate cultures (e.x. Belgium). Many fall in between, with cultures that are different but not necessarily separate (I think Italy falls into this category). Quite a few don't even really correlate with any culture (many of the post-colonial nations in Africa, for example).
My overall point here is that I don't think you can judge whether or not a region should separate just by trying to measure the cultural difference between them and the rest of their country (which is pretty much impossible to do objectively, anyway). Sometimes culturally different areas are best served by remaining party of one country, and other times they are best served by separating. Sometimes the answer is some sort of devolution/regional autonomy. But all of that takes into a lot more than just how culturally different one area is from another area, including the history of the areas, the economics of separation vs unity, the configuration of the government, the rights given to residents of the regions that want to separate, etc.
EDIT: I'm not disagreeing with you, btw, I think this actually supports the point you were making.
